1. Field of Invention
The invention relates to a process for the preparation of 2,6-xylidine by amination of 2,6-dimethylphenol in the vapour phase with ammonia in the presence of an aluminum oxide catalyst.
2. Discussion of Prior Art
It is known to react phenols in the vapour phase with ammonia under pressure on catalysts containing aluminum oxide to give the corresponding anilines (U.S. Pat. No. 1,935,209, U.S. Pat. No. 2,013,873, U.S. Pat. No. 3,272,865, German Auslegeschrift No. 2,026,053 and German Offenlegungsschrift No. 2,003,842). Although the publications mentioned list a large number of phenols as suitable starting compounds for the reaction, only a few phenols, in particular phenol itself, m- and p-cresol and 3,5-xylenol, are employed in the examples given in the publications (U.S. Pat. No. 3,272,865, U.S. Pat. No. 1,935,209 and U.S. Pat. No. 2,013,873).
According to German Offenlegungsschrift No. 2,516,316 difficulties occur in amination reactions with substituted phenols. It is thus mentioned, for example, that when m-cresol is used, up to 10% of by-products are formed. German Offenlegungsschrift No. 2,516,316 recommends the addition of toluene in order to avoid these disadvantages. 2,6-Substituted phenols have not yet been employed at all in the amination reactions known from the state of the art. The preparation of 2,6-xylidine was hitherto carried out by reacting aluminum tris-(2,6-dimethyl-phenolate) with ammonia at a temperature of about 200.degree. to 500.degree. C. (German Auslegeschrift No. 1,933,636).
In another process (German Auslegeschrift No. 2,208,827) 2,6-dimethylphenol is reacted with ammonia at 200.degree. to 400.degree. C. in the presence of a hydrogen transfer catalyst and in the presence of water and catalytic amounts of a cyclohexanone to give 2,6-xylidene.
On the basis of the steric hindrance by the 2,6-substituents and of the teachings known from German Offenlegungsschrift No. 2,516,316 that o-substituents impede the amination reaction and give rise to the formation of by-products, it is understandable that the amination reaction which is in itself known has not been applied to the preparation of 2,6-xylidine and that the expensive processes according to German Auslegeschrift No. 1,933,636 and German Auslegeschrift No. 2,208,827 have been turned to for the preparation of 2,6-xylidine.
If 2,6-dimethyl-phenol is reacted under the known amination conditions, a comparatively low conversion is indeed also found, and the formation of by-products, in particular isomerization and disproportionation with formation of aniline, o-, m- and p-toluidine, isomeric xylidines and trimethylanilines, is observed to a considerable extent. Thus, the reaction of 2,6-xylenol with ammonia in the gas phase under the amination conditions described in German Auslegeschrift No. 2,026,053 gives, after 50 hours and with 95% conversion, a product mixture containing only about 50% of 2,6-xylidine.
Apart from the loss in yield resulting from the poor selectivity, the considerable amounts of isomeric 2,4- and 2,5-xylidines, in particular, present great difficulties during working up, since 2,4-dimethylaniline, above all, cannot be separated off or can be separated off only with a very high expenditure on distillation.
Even increasing the pressure to the upper limit of 70 bars given in German Auslegeschrift No. 2,026,053 does not give rise to an improvement either in the conversion or in the selectivity when one of the catalysts used in German Auslegeschrift No. 2,026,053 is employed.